Matric potential responder improvements

ABSTRACT

A matric potential responder which includes a liquid absorbing swellable nonliquid material ( 115 ) held within a housing ( 114 ) of a ceramic material environ such as soil, to the material ( 115 ), being subject to a compressive force such as a spring ( 116 ). This uses balance of compressive force as compared to tension of the soil matric potential being used to control a position of a valve ( 119 ) which is arranged to effect an output such as control of a mains water valve ( 110 ).

FIELD OF THE INVENTION

This invention relates to an apparatus and a method for effecting aresponse to the matric potential of a medium such as soil which can beuseful as a measurement or as a control for water supply.

This then may include a matric potential responder, an actuator for usein a responder, a sensor for use in a responder and method of measuringmatric potential.

BACKGROUND OF THE INVENTION

It is known to measure some factors of soil to establish some guide toan amount of water that might exist in soil to facilitate growth ofplants in that soil.

There are accordingly devices measuring the electrical resistance of thesoil, the electrical impedance of the soil or even the dielectricconstant of the soil, each of which measures are useful in some casesbut do not however provide a measure of matric potential.

Plants derive access to moisture in soil by overcoming matric potentialin the soil or as it is sometimes termed “soil suction”.

Such a characteristic is currently able to be measured by a tensiometerbut there are significant difficulties with any known tensiometer. Onesuch difficulty but not the only difficulty is that a tensiometer is notuseful outside of a relatively small range of matric potentials (zerokilopascals through to 80 kilopascals) which range is substantially lessthan would be useful to assist in assessing water availability forplants.

An object of this invention is to provide a further method and apparatusby which matric potential can be measured or used to assist in wateringfunctions for plants which can at least provide an extended range ofmeasurement or response and which, at relatively small cost, can providewith reasonable accuracy such response or measurement.

It is previously known to use a water swellable material which when putin contact with water will swell and then use this effect to close avalve supplying water.

Hitherto, most such proposals have described a water swellable materialas a material that will, in the presence of sufficient water, swell sothat given enough time and enough water the extent of swelling will besufficient to close a valve against significant resistance or trigger arelease against a substantial amount of resistance, with the triggerthen further releasing some valve or other operation for control ofwater supply.

Such previous applications then seem to treat the use of such materialas simply a detector of the presence of water so that with water presentand given sufficient time and generally total water inundation thischange in status of the material from not swollen to swollen or theopposite, can then be used to trigger some control either directly orindirectly.

In serious water use applications such an approach has seriousdeficiencies. It does not, for instance, provide for commencement ofwatering in conditions which are ideal for the plant where any dryingbeyond perhaps matric potential of 150 kilopascals is undesirable. Ifone cannot measure this then it is an open question as to when a startmight occur and this is simply unacceptable for serious watering ofplants.

Such devices that I have previously seen proposed, then, are not set upto or do not respond to take account of the matric potential of themedium to an extent that makes them useful for serious control ofwatering.

SUMMARY OF THE INVENTION

I have found that it is possible to measure matric potential using waterswellable non-liquid materials and further more use these materials tothen usefully control supply of water for serious watering purposes.This discovery results from an understanding of what seems to be a basiccharacteristic of such materials. If the material is exposed to aselected value of matric potential then this will effect an internalsuction effect within the whole of the material. If we then apply ameasured external force to the material which has the effect of applyingthroughout the material a compression, then soil suction effect ormatric potential within the material will be offset by this compression.

In one form this invention can be said to reside in a matric potentialresponder which includes a liquid absorbing swellable material whichwill exhibit an internally developed increase or decrease in expansivepressure within the material in response to the matric potential ofhydraulically connected soils or other medium, means applying and/ormaintaining a compressive pressure to the said material, and meansadapted to effect an output in response to changes in respect of theinternal pressure of said material.

In preference, the material selected and the compressive pressureapplied are such that the responder will provide an output to detectedmatric potential within the range of from 50 kilopascals to 300kilopascals.

In preference the matric potential responder includes a housing at leastone part of the housing being porous to provide for the effect of matricpotential within liquid to be able to transfer therethrough, and withinthe housing, a liquid absorbing swellable material which will exhibit aninternally developed increase or decrease in expansive pressure withinthe material in response to the matric potential of the hydraulicallyconnected materials, means to apply compressive pressure to the saidmaterial within the housing, and means adapted to effect an output inresponse to changes in respect of the internal pressure of saidmaterial.

In one preferred form, there is a ratio of hydrogel to container orhousing volume which will result in an internally generated pressure sothat with unlimited access to water such that pressure will be 200kilopascals. There is provided a piston with a limit to its outwardmovement subject to a resilient pressure such that an externally appliedpressure will be 100 kilopascals to the material.

When the hydrogel is exposed to a matric potential of greater than 100kilopascals, the piston will displace the material to an extentresulting in a change of volume and inward movement of the pistondependent upon the extent of the soil matric potential to which thehydrogel is exposed. This movement can be utilized by connecting it toother mechanisms for indicating and control purposes.

With a housing defining a substantially fixed volume, a change in matricpotential of swellable material held confined by the housing will beexhibited by a change in internal pressure within the material. Such achange in pressure is generally linear with respect to the change inmatric potential. (This can be compared to difficulties of using avolume change which will be generally non-linear with respect to matricpotential).

In preference, the pressure applied is such that the externally appliedpressure will be such that the physical pressure other than through thematric potential change will be approximately 300 kilopascals.

In a further form, in preference, there is a pressure transducer withinthe material such that any change in pressure within the material willbe directly detected by the electrical transducer which in turn then canprovide an electrical effect in accordance with the detected magnitudeand/or change in pressure.

In another preferred arrangement, the material is subjected to a head ofliquid by being separated from the material by a flexible membrane andsuch then that the head of liquid which can be open to atmosphere, willthen exhibit a change in height in response to a change in pressurewithin the material which in turn depending upon the characteristic ofthe particular material, will expand or contract in accordance with theexternally detected matric potential of the medium within which it islocated.

In preference, such liquid to exhibit a change in pressure should be ofhigh specific gravity so that liquid mercury can be used so that theupper most head of the mercury can then be calibrated in accordance withchanges in expansion of the material which in turn are caused by achange in internal pressures in accordance with the balance of matricpotential effected.

In such an example, the liquid itself provides the first pressure toestablish a physical state of the material so that any externallydetected matric potential will reduce the internal pressures within thematerial proportionately.

For instance, if the material is pressed to be 300 kilopascals, and itis put into hydraulic communication with soil having 100 kilopascalsmatric potential, then this will reduce the volume of the material to a400 kilopascals volume.

Such a change can then be detected by detecting the volume change of thematerial for example by a simple sight glass.

In preference, the housing including or being totally comprised of aporous material has for its purpose to provide for a hydrauliccommunication which will allow the transfer of matric potential betweenthe materials on respective sides of the porous material and to ensure astable volume so that the volume change of the swellable material willbe reliably transferred to the measuring means.

Baked and unglazed ceramic materials conventionally exhibit thischaracteristic and in experiments so far have shown to be appropriate.

Ceramic material is substantially rigid so that changes within theinternal pressure of the material held with at least one part againstthe ceramic will not unduly deflect the ceramic so that the position ofother portions of the material can be used to gauge the effect of thechange in internal volume.

The ramifications of these steps and features are very high indeed.

Conventionally, a tensiometer is simply filled with water and when in amedium exhibiting more than 80 kilopascals suction, this will cause thewater within the housing of the tensiometer to potentially break into avacuum status dependent upon the vapor pressure of the water at the timeand therefore be no longer useful to measure any different or extendedsuction range.

Growing plants mostly access water when held within a range of matricpotential from 0 kilopascals to 300 kilopascals (they can lesscomfortably access water from soil at over 1,500 kpa) within the soil sothat the one conventional technique of measuring soil suction is notuseful outside of a small portion of the range of suction that should beable to be detected.

The discovery of this invention is that we need no longer be subject tothe difficulties associated with the previously known simpletensiometer.

Accordingly this is because liquid preferably within an appropriateporous housing is held by a medium which, of itself, exhibits aninternally compressive pressure and is exhibiting this effect by reasonof molecular attraction within the structure of the material.

It will be readily appreciated that many different approaches can now beused to apply this concept to a number of various responders both forthe purpose of effecting an output that of itself can be used to performwork by way of directly changing the open or closed status of a valve orin another form, have the pressure directly detected to effect either anelectrical or visual output in response to such detected pressurechanges in response to changes in the matric potential effect.

It is to be understood that what is being described is different fromsomething which merely uses a material which will expand when wet andwhich will contract when dry.

The material that is to be used is something that can exhibit astructure which will provide a force with the effect that water will beheld against an externally applied force and will give up that waterwith the extent of that force.

It has been found that a material which is a solid hydrogel can be usedas one example, and in another example, a material which can absorb somuch water as to be essentially fluid like, nonetheless can also exhibitthe necessary characteristics.

If the material is to be solid, in preference, there are arrangementsfirstly to ensure that there is an effective hydraulic connection to thematerial from the porous barrier and further the solid material isarranged so that any pressure changes internally will result in apredictable expansion or contraction or change in pressure or in theseall which can then be used to accurately assess the effective result.

In practice this has meant that the material can be in the form of thindisks which are flat and stacked one upon the other with sheets ofinterlocking fibrous material therebetween with solid plates such asmetal, plastic or ceramic plates therebetween. Most of the transverse tothe planar direction of the disk shape will expand or contract inresponse to the detected change in matric potential with the planardirection expansion being inhibited substantially to an extent that thevolume is constrained howbeit with interleaving fibrous sheets.

The interleaving wicking material is arranged to extend to a side of thestack to be in contact with the material for matric potential transfer.In another approach, there is a solid material which is however dividedwith a number of slots which are filled with wicking material or atleast material that will transfer liquid to provide a matric potentialcommunication.

For a better understanding of this invention it will now be describedwith reference to preferred embodiments which shall be described withthe assistance of drawings wherein

FIG. 1 is a cross sectional view of a conventional tensiometer,

FIG. 2 is a first embodiment showing a constrained water swellablematerial within a housing and a manometer pressure respondingarrangement,

FIG. 3 is a second embodiment which is substantially the same as in FIG.2 except there is a Bourden tube pressure measuring device providing aresponding arrangement,

FIG. 4 is a third embodiment with hydrogel constrained along one side bya porous ceramic and against an opposite side a slug of pressuretransmission material to an electrical pressure transducer,

FIG. 5 is a schematic cross sectional view of a responder arrangementbeing a fourth embodiment including a soil matric responder connected toan actuator controlling a water distribution arrangement,

FIG. 6 is an enlarged view of the actuator as shown in FIG. 5,

FIG. 7 is a cross sectional view of a fifth embodiment of an integratedresponder water flow controller,

FIG. 8 is a cross sectional view of a sixth embodiment of an alternatesensor that can be used in the arrangement of the fourth embodiment,

FIG. 9 is a cross sectional view through a further sensor that can beused in the arrangement of the fourth embodiment,

FIG. 10 is a cross sectional view through a responder in accordance witha seventh embodiment showing this when responding to a dry state,

FIG. 11 is a cross sectional view through the sensor along the lines of11—11 of FIG. 10,

FIG. 12 is a cross sectional view through the responder in accordancewith the seventh embodiment as shown in FIGS. 10 and 11 but showing thiswhen responding to a wet state,

FIG. 13 is a cross sectional view through the sensor of FIG. 12 alongthe lines of 13—13,

FIG. 14 is a schematic view in cross section of the apparatus used totest the responses of various matric potential responding materials,

FIG. 15 is a graph of a relationship plotted between the hydrogel(polyacryalmide) volume and a load applied to that hydrogel inKilopascals,

FIG. 16 is a graph of a result matching % volume change with totalpressure applied to the hydrogel material (polyacrylamide). Sensorvolume changes will decrease with the spring loading of the piston sothat if the additional load increases by 50 kpa then an additional 50kpa of change in sensor pressure will be required to effect a 10% changein volume.

FIG. 17 is a graph as in FIG. 16 showing that there is some advantage insensitivity if an offset pressure for a given volume of sensor(polyacrylamide) is approximately that of the matric potential to bemeasured in devices measuring 20 volume changes,

FIG. 18 illustrates changes in response in some hydrogels (specificallypolyacrylamides) over a two week period of repeated cycle times in whichthe first run was at the beginning and each reading showing a successivereading from one to two days apart to the last reading which was afterfourteen days.

These results show a decrease in pressure sustaining capability of 50kpa over the period of the test. This indicates that this material isless useful for longer term installations.

FIG. 19 illustrates how the responder material namely hydrogel willrespond to both suction and pressure without distinction. This graphshows specific results using polyacrylate which as a material has shownmore retention of its response effect over a period of time and would bethe currently preferred material.

Referring to FIG. 1, this is a description of the prior art which is aconventional tensiometer where there is a closed tube 1 of a ceramicmaterial which has within it water 2 and attached in a sealedrelationship is a conduit 3 connected to a Bourden tube pressuremeasuring device 4.

The tube 1 is placed beneath the ground shown typically at 5 and theextent of suction of the soil will effect suction of the water 2 throughthe hydraulic communication provided by the ceramic tube 1 and this willbe recorded as a suction pressure by the measuring device 4.

Typically, this type of device is limited by the vapor pressure of thewater 2 which is about 80 kilopascals. Another difficulty is thepossible entry of air through the ceramic. Both of these problems areovercome by the present invention.

A first embodiment of the current invention is shown schematically inFIG. 2. In this case there is provided a housing 10 which has within ita disc of hydrogel 11, a ceramic disc 12 on one side and separated by apliable membrane 13, liquid mercury at 14.

A liquid mercury is connected through conduit 15 to a vertical tube 16,the upper end of which at 17 is open to atmosphere and the position ofwhich is measured by calibrations shown at 18.

In this case the single disc of hydrogel is an ultra-high moleculeweight a homopolymer of 2-hydroxyethyl methacrylate which is referred tobriefly as “2-hema”.

In this first embodiment then, the disc of 2-hema is subjected topressure through the pliable membrane 13 of the head of mercury which ischosen to apply an appropriate pressure onto the 2-hema material so thatwater suction with which it will be in communication through a hydraulicconnection through the ceramic material 12.

Because it has been discovered that there is a fixed relationshipbetween pressure, volume and water content and that water content can bedirectly related to the matric potential of a medium with which it is inhydraulic communication, the hydrogel 11 in this case then will swellwhen communicating with a matric potential that is less than thephysically applied pressure from the head of mercury. In this way then,there can be effected an output which is in relation to the head ofmercury against the calibration which can be directly correlated withthe matric potential of soil within which the housing 10 is located.

The calibrations in this embodiment are such that there will be shown arange of matric potential which will extend at least through the rangeof from 0 kilopascals to 100 kilopascals. The “2-hema” material in thiscase is a solid plastics material and it is held within the housing 10by side walls 19 so that any expansion or contraction is constrained byreason of the close fitting sides and therefore expansion or contractionat right angles to the planar orientation of this material will bedirectly proportional to the increase in volume of the material.

FIG. 3 is a second embodiment which has substantially the same elementsas in the first embodiment including a housing 20, a disc of hydrogel 21which is the same material as used in the first embodiment namely2-hema, and a pliable membrane 22.

A measure of the volume pressure relationship within the hydrogel 21 andthe application of an applied compressive force is achieved in this caseby use of a Bourden tube pressure gauge 23 connecting through ahydraulic link 24 to the chamber 25 within the housing 20.

There is a substantially incompressible liquid within the conduit 24 sothat the traditional Bourden tube which has a flattened tube of springmaterial in a curved shape will then both enable the maintenance of aselected degree of pressure to be set through the fluid by calibratingthe pressure gauge, and then allowing the pressure gauge to respond tochanges in pressure which in turn will reflect changes in volume ascompared to the applied pressure balanced against the detected matricpotential of the medium within which the housing 20 is located so thatthe ceramic 26 can be in hydraulic communication with the medium.

FIG. 4 is a third embodiment in which there is a housing 30 within whichthere is an implanted piezo resistor 31 which is a known device whichwill provide a voltage output through its respective output leads 32which voltage output is directly proportional to any applied pressure.

In this case, a hydrogel is again chosen which is 2-ultrahigh moleculeweight homopolymer of 2-hydroxyethyl methacrylate of type G55 sold byBenz for the manufacture of contact lenses.

The material in the dry form is shredded into fine particles and is thensaturated with water.

The material in this form shown at 33, is captured within a ceramic cap34 which is then upside down to that position shown in FIG. 4. Screwsincluding at 35 and 36 are used to squeeze excess of the material frombetween the mating surfaces of the housing 30 and the ceramic 34 and asthis squeezing continues, apply a set pressure to the hydrogel 33.

Filling the space between the hydrogel 33 and the piezo resistor 31 is asilicon gel which is molded into the cavity within the housing 30 and issuch that it will convey relatively directly, any pressure changeeffected by reason of even slight change in pressure in the hydrogel 33.This means that this is an almost entirely pressure responsivearrangement.

In a further embodiment not shown specifically in the drawings thealcosorb material has been replaced by 2=ultrahigh molecule weighthomopolymer of 2-hydroxyethyl methacrylate of type G55 as describedpreviously for the manufacture of contact lens which h&s shown someadditional advantages. The material is precision machined to apredetermined size and placed in a cavity of the previous example inplace of the alcosorb. The ratio of the volume of hydrogel to the volumeof cavity determines the gel pressure in its saturated state.

The result of the arrangement shown then is that a capsule which canhave very small dimensions indeed, can be located in any position wherehydraulic communication can be effected with the ceramic material 34 andbecause there will be no necessity for a substantial change in volume tocause any change in pressure to be detected, such a device becomes verysensitive and very fast acting in response to changed conditions indeed.

The pressure sensor used in this embodiment has been obtained fromSenSyn Inc of the USA. and is one of their SX series devices which havebeen modified in a way described to make it directly responsive to soilmatric potential.

More particularly, the range of soil matric potential that can bedetected and measured directly in this way is not limited to the rangepreviously available only through a tensiometer and in trials conductedso far, a device of this type can be used to measure soil matricpotential from close to 0 kilopascals to 10,000 kilopascals.

At the least then it provides a device to measure matric potential orsoil suction as it is often referred to, in a range that is greater than50 to 80 kilopascals.

Now referring to FIG. 5, this is a fourth embodiment which shows both asensor and an actuator somewhat schematically where the actuator isarranged to control flow of water the extent of which then can bedetermined by a change in soil matric potential which is able to bedetected by the sensor.

Describing this arrangement in detail, the sensor 40 includes a housing41 which is made from a ceramic material which, as with the precedingceramic materials described, are chosen and have a porosity such thatthey can provide good hydraulic communication with a soil within whichthey are placed.

In this case, the ceramic 41 is in the form of a cylinder having howevera number of inner cavities which are each filled with fluidiz Alcosorbmaterial as used in the last embodiment.

The fluidized Alcosorb material fills the plurality of cavities showntypically at 42 and is separated from chamber 43 by a membrane 44.

The membrane 44 in this case is cylindrical and is closed at an outerend by stopper 45 and at the other end by stopper 46 which also sealablyconnects conduit 47 to actuator 48.

The actuator 48 has a central plunger 49 adapted to move along a bore 50within a housing 51.

The position of the plunger 49 is predominantly established by pressureexerted by spring 52 which acts to lift the plunger 49 and this iscounted by any developed pressure within the conduit 47 delivered by wayof the noncompressible liquid into chamber 53,

This is kept sealed from the area providing the spring pressure by apliable membrane 54.

The result of selected balance of pressure will be that the plunger 49can cause the valve head 55 to be closed on valve seat 56 which willthen interrupt flow of liquid through conduits 57 and 58.

The output of 58 is into a number of distribution heads typically andschematically shown at 59.

An effect of this arrangement is that a sensor can be located remotelyfrom a directly operating actuator and that advantage can be achievedfrom the expanding volume as a result of increasing pressure within thehydrogel such that work can be effected by such a detected change.

A first setting of pressure on the hydrogel can be effected by choice ofspring rate and tension in spring 49. (the loaded pressure of the springin this embodiment is 2.2 kgs at a length of 25 mms and its unloadedlength is 150 mms). (This means that there will be a relatively smallchange of pressure over modest displacements of the spring length sothat this with the area of the diaphragm directly determines theoperating range of the device with the spring movement). Accordingly thedevice can be set to operate over a selected range of pressures. Inparticular such an arrangement can work within the range of detectedsoil suction between 50 kilopascals to 300 kilopascals although it canalso be used to operate in response to a wider range than this ifneeded.

An advantage of the fluidized hydrogel in this application is that itcan be inserted into the labyrinth internal shape of the ceramic readilyinsofar that it acts like a fluid and further, there is a high degree ofsurface area as compared to hydrogel achieved in this arrangement whichmeans that response times to any change in external environment can bequickly transferred and detected by the internal materials and further,can effect a sufficient change for substantial work to be output.

It will be noticed that the housing 48 is similar to an electrical coilof a type used conventionally as a solenoid controller for irrigationcontrol valves so that it will be seen that the actuator in this casecould directly replace such an electrically operated solenoid.

FIG. 6 is an enlargement of the actuator as shown in FIG. 5 and severaladditional features can be further therefore described including theconduit 60 providing atmospheric access to the chamber 50, the O-ring 61which keeps water within the main supply conduit 57 sealably separatefrom the chamber 50, and there is shown an arrangement of springs 62 soas to transfer an effective pressure onto the valve head 56.

The feature of providing a closed liquid connection between the sensorand the actuator allows for a translation of an amount of volume changeand pressure into any other combination of volume and pressure in aconvenient way. It means also that results from the sensor can betransferred to a remote location without the need for a supply ofelectrical power.

FIG. 7 is a further embodiment which has an actuator very similar to theactuator described in the last embodiment but to which there is directlyattached a sensor in accordance with the invention. The actuator thenincludes a housing 70 with a central rod 71 passing through a chamber 72which is open to atmosphere as shown through conduit 73.

The rod 71 has at its end an arrangement 70 again providing forcontrolling of the position of valve head 75 with respect to valve seat76.

This in turn then controls the flow of liquid from 77 through to 78.

The difference here however is that there is attached to the housing 70a sensor housing 78 which holds a disk of porous ceramic 79 which holds,between itself and a membrane, a fluidized hydrogel 81 prepared in themanner described previously where Alcosorb was the material used.

As will now be seen, hydraulic communication can be achieved through theceramic 79 into the hydrogel 81 and this will be sealed against transferof liquids but nonetheless will transfer a change of pressure resultingin a volume change against spring pressure exerted by spring 82 of plate83. This then allows for a setting of the spring with the quantity ofmaterial of hydrogel such that a preset pressure of perhaps 200kilopascals can be applied which will then allow for the hydrogel torespond to changes in matric potential of about 100 kilopascals at whichpoint the opening sequence will begin to control the passage of watereffecting a watering program.

FIG. 8 is a cross-sectional view of an alternative sensor where thematric potential responding material is a solid hydrogel in this case2-hema. In particular the sensor 90 includes an outer cylindricalhousing 91 of a porous ceramic material adapted to provide a hydrauliccommunication between the environment in which it is inserted and aninner matric potential sensitive material shown at 92, The respondingmaterial in this case is a cylindrical slug of 2-hema with a hollowcore.

In order for the solid slug to fit readily within the ceramic outercasing 91, this has a matching shape with however a plurality ofcircumferential slits shown typically at 93, each of these slits beingfilled with a wicking material which is chosen to be able to transfer inhydraulic manner matric potential from externally the sensor to thegreater cross-sectional area of the responding material 92 in this way.

The cylindrical casing has an end cap 94 and a top cap 95 which holdstherebetween a pliable membrane 96 which provides liquid isolationbetween the responding material 92 and an inner chamber area 97.

The inner chamber area 97 is mostly filled by a rod 98 which however hasa conduit 99 which thereby provides communication for liquid held in aclosed passageway for connection to an actuator.

As with the earlier embodiments, the matric potential respondingmaterial is chosen so that with appropriate application of hydraulicpressure, it can itself effect a volume pressure response which can beused to directly control an activator or of course any other devicewhich it is useful to control in response to detected matric potential.

The range chosen for this particular embodiment also extends from 50kilopascals to 300 kilopascals matric potential of the soil if thesensor is placed in the soil. FIG. 9 is a cross sectional view of afurther sensor that can be used in conjunction with the fourthembodiment.

This then includes a non-porous housing 100 which has a cylindricallyspaced cavity 101 in which there is a ceramic disk 102 which holds inplace a slug of hydrogel of 2-ultra-high molecule weight homopolymer of2-hydroxyethyl methacrylate which is fitted to nest tightly within thecylindrical shape of the cavity 101. There is a pliable membrane 103which is captured between mating surfaces 104 of the housing 100.

The ceramic disk is rigid and holds the slug of 2-ultra-high moleculeweight homopolymer of 2-hydroxyethyl methacrylate so that any change involume will reflect in a proportionate change in the pressure againstthe membrane 103 and through this, against generally incompressiblefluid 105 which is connected through conduit connections such as at 106to an actuator where there will be applied a selected extent of pressurewhich is therefore applied to the 2-ultra-high molecule weighthomopolymer of 2-hydroxyethyl methacrylate material. Any change in anextent of soil suction transmitted through the ceramic disk 102 istherefore transformed into a change in pressure in respect of the fluid105.

Now referring to FIGS. 10 through to 13, these show a further embodimentwhich includes both an actuator 110 and a sensor 112.

The sensor 112 is arranged to control water flow passing through thesensor 112 out to a drain 113. The rate of flow is changed by varyingresistance to flow provided by an opening of a valve (the actuator 110)to a mains supply of water so that with such a change to water flow thisalters a balance of pressures in the actuator 110 and is arranged tochange this from open to closed or from closed to open as the resistanceto flow of water changes in the sensor 112.

In detail there is in the sensor 112 a non-glazed ceramic housing 114which is therefore porous and will provide hydraulic communication (thatis it will provide transfer of soil suction) to a material within thehousing and especially the number of bores 115 passing deeply into thehousing shape. These are 5 filled with an hydrogel specifically a crosslinked polyacrylate (purchased from CIBA as Ciba Gelling agent 31). Thisis still a solid or gel but in this form is able to conform to a morecomplex shape such as that being shown in this example in the housing114.

The hydrogel located within the respective bores 115 is held generallyunder a pressure which is applied by reason of spring 116 which issupported so as to press against a transversely extending pin which isarranged to pass through piston 119 and so that the spring and themembrane will act directly against the hydrogel in the bores 115.

This pressure being applied specifically by the spring 116 will, asappropriate, be opposed by a decreasing extent of soil suction whichwill hydraulically communicate through the porous interstices of theceramic housing 114.

The pin 117 is embedded within piston 119 which has an aperture 120passing through it, through which the arm 121 passes through therebyengaging against the bottom of the pin 117 and thereby providing a forcedepending upon the spring rate and the extent to which the shaft 138 andthe spring 116 is tightened or loosened whereby to alter the springforce.

The sensor 112 as a whole is supported by a general housing 122 whichhas a conduit 123 connecting to the actuator 110 and a drain 113.

There is a bellows 124 which is arranged to have its inside volume incommunication with liquid passing through conduit 123 and in particulartube 125 which is arranged so that, with an increase in pressure withinthe conduit 123, there will be an effective increase in pressure withinthe bellows 124. As this is made up of two parts which are resilientlyretained in a first retracted position, this will be urged to expandwith increased relative pressure.

Such an increased relative pressure will occur when the piston 119 isurged into closer relative positioning with respect to seat 126.

The effect then is that with a modest increase in resistance to waterflow through conduit 123, there will be caused firstly an increase inexpansion of the bellows 124 which will further raise the outletprovided by the seat 126 and this in effect causes a snap actionclosure.

Such a closure then effects a total closure of liquid passing throughconduit 123 which then causes the pressure within chamber 130 in theactuator 110 to increase in that there is no longer a draining of liquidfrom this chamber through the conduit 123. This in turn then will causea build-up of such pressure to effect a closing of the main valve 131thereby closing a main supply.

Accordingly, there is provided a slaving arrangement operated by thesensor 112 which is efficiently caused to have a snap action dependingupon the status of pressure relatively provided by hydrogel within theceramic housing 114.

The actuator 110 is a relatively standard mains control valve whichincludes a mains water passageway 132 which passes through seat 133 tooutlet 134.

Conduit 135 is a feature of a standard valve which is closed off forthis operation and replaced by vent 113. Accordingly liquid feedingthrough gap 137 will either build-up or reduce pressure within thechamber 130 as compared to the rate of discharge through the outlet 133.

This comparative ratio is, of course, varied by providing additional orreduced lead off externally and hence the sensor 112 can be caused to bea very effective controller.

A few points that are of note include the fact that the piston 119 doesnot respond and need not respond through a complete range of movementsin direct response to soil matric potential but only to that extent ofmovement which is in the range of the spring movement available andhence can be caused to operate at a selected extent of soil matricpotential.

FIG. 10 indicates the relative positioning of the moveable parts whenthe soil matric potential is high and therefore the piston 119 is keptraised and accordingly it will be seen that the main valve 131 is openwith therefore passage available of mains water through the mainsconduit 132.

FIG. 12 illustrates the opposite position in which there is a higherdegree of moisture in the soil and there is, therefore, a greater degreeof saturation which is transmitted hydraulically through the porousceramic housing to the hydrogel which in turn then is under moreexternally applied pressure through the spring 116.

This, however, effects the closure of the conduit 123 and hence thebalance of pressures operating within the chamber 130 is such that themain valve 131 is closed thereby blocking mains water supply.

In FIG. 13, this illustrates, from a plan view, the spring 116 and itsrelationship with the piston 119 and, as will be seen, the spring 116 iswound around a rod 138 which is held under a friction grip by grip 139but such that the rod 138 can be rotated against resistance about itsaxis so that it can wind tighter or looser the spring 116 about the rod138.

Such a setting can be controlled from an external access to aperture 140by an allen key.

With the arrangement described, a member 141 is simply a plug that isable to be sealably screwed into the top of the actuator valve 110 whichis connected with a supply of liquid through to the sensor 112.

This plug 141 simply replaces a standard electrical solenoid of a typethat is relatively standard as used in many irrigation installations.

Accordingly there is provided a controller for such valves which canaccurately respond to a selected extent of soil matric potential whichdoes not require an electrical supply and which can be connected at anylocation where an appropriate conduit can be connected for transfer ofliquid flow.

One of the advantages of using a ceramic housing where there are anumber of separate bores is that this allows for substantialdistribution of the hydrogel material where the housing itself willdistribute through its porous hydraulic communication, the effect of thesoil matric potential to a high surface area available of hydrogel.

Further, the fact that the housing itself is a single element which canbe cast, means that such a unit can be economically manufactured forthis application.

One of the advantages of having the snap action provided by theexpanding or reducing bellows is that this can mean that when a mainswater supply is being controlled by the sensor as described, then if thewater supply is being distributed by, for instance, a sprinkler thespread of water will be maintained relatively constant until the sensorsnaps into a closed position at which stage the water flow will bestopped and, of course, the sprinkler will not be operated at a pressurewhich would be wasteful of water or uneconomic.

This is not always necessary in the case, for instance, of a dripperalignment where lower pressures of water can be acceptable but, even insuch instances, it is considered better generally to have either a fullflow mains pressure or no flow at all. This effective in reducingfouling problems in water supplies containing small particles.

FIG. 14 illustrates, in a schematic way, a test rig that has been set upto test the characteristics of appropriate materials for the applicationof the water soluble material.

Some materials are discovered as being better for the application thanothers.

Currently all hydrogels that have been tested have been able toillustrate the effect which is that they will exhibit a change ininternal pressure which is in response to externally applied mechanicalpressure and an extent of soil suction to which the material isconnected.

This effect then results in a relative internal pressure result whichcan either be read directly or can be determined by measuring an extentof volume change against internal resilient or other resistance.

This then distinguishes this concept from any case where there is merelya slug of material which is known to swell or to contract with exposureto water.

Here in FIG. 14, there is now described a test rig by which materialscan be tested as to their appropriateness for the application.

Accordingly, there is a supply of air at pressure through valve 150 andpressure regulator 151 with, however, an accurate pressure measuringdevice at 153 and an accurate displacement measuring device at 154.

The test rig itself includes a cylindrical housing at 155 with an uppercap at 156 and a lower cap at 157.

The ceramic housing at 158 being shown in this instance, is the same asshown in the last described embodiment, with a number of deepboresdrilled into a porous ceramic material and this is filled with thehydrogel or other material on test shown generally at 159.

Soil matric potential is simulated by effecting a suction throughconduit 160 which, however, is maintained through applying the lowerpressure into space 161 so that the reduced pressure is applied throughreduction of pressure in water 162 which then is applied as a saturatedliquid surround at 163 which applies the selected extent of suction.

The accurate extent of suction is determined by accurate mercurymanometer 164.

Now, as we apply a selected extent of pressure such as, for instance,300 kilopascals through the conduit 165 in the first instance and thenapply a change in effective suction, the resulting change in pressure aswell as the extent of change in volume can then be accuratelydetermined.

As can now be seen, the degree of tension applied can be read frommercury manometer 164.

To ensure that a match of calibration is achieved for a degree ofpressure applied, the pressure is determined by the mercury manometers153 and 164.

The change in volume of any hydrogel or other material on test is readby a change in the level of the sight glass of water within 154 as wateris moved into a chamber behind diaphragm 156 and into space originallyoccupied by the material on test. The ceramic disk 156 a provides asupport for the diaphragm 156 in a back pressure situation.

Using this test rig, it has been discovered that the material on testespecially hydrogels, are responding to the differential pressureapplied either side of the ceramic interface and that there has been nodistinction between the levels of positive and negative pressure appliedin regard to the degree of volume change of the material on test.

From this, I have deduced that the method of loading the material ontest has been especially relevant to accurate measurement. It has beenfound, for instance, that an applied pressure to the material on test isa useful way of setting a measuring range of a sensor as the pressureapplied to saturated material on test at 0 kpa tension determines thedegree of tension that would need to be applied to the material on testin a chamber of fixed volume before the internal pressure of thematerial on test reached 0 KpaG.

When a constant pressure is applied to the material on test, while it issubjected to tension forces, the volume occupied by the material willreduce by a same amount for say 100 Kpa tension as for an increase of100 Kpa in the pressure applied. I therefore say that by using such atechnique, a person who wishes to establish an appropriateness of aparticular material for the purpose can now determine this through thisaccurate but relatively simple test rig device.

FIG. 15 is a graphical representation of a sensor material in volume incubic centimeters against the load applied in Kilopascals, the result ofmeasurements of the matric potential material namely the hydrogels whichhave been used and it illustrates the way in which there is arelationship between the kilopascals load and the volume of the sensormaterial.

I now refer to the graphs in FIGS. 16, 17.

In FIG. 16, this illustrates the tests that have been conducted, showingpercentage volume change as compared to total pressure for a firstselected material on test namely polyacrylamide.

The method of test has been as follows;

Water is added to a selected quantity of polyacrylamide particles sothat it has more water than would be required to support 100 kpa. Thematerial is placed in a container which has an outlet for the materialin the water ingested state at its bottom, and an inlet for air at thetop of the container.

The outlet is connected to a sensor cavity of a porous housing which isat first filled with water to eliminate air. When pressure is applied tothe top of the hydrogel at 100 kpa, the material will enter the cavitydisplacing the water.

Water will then be expelled from the hydrogel material through thepermeable walls of the cavity until the material remaining will support100 kpa without further loss of water. A combination of suction andpressure are applied to the hydrogel material, suction from outside thecavity and pressure by way of an isolating diaphragm to the inside ofthe cavity. With combinations of pressure and suction in excess of 100kpa additional water will be expelled from the material with a resultantchange in volume indicated by a sight glass connected on the outside ofthe diaphragm.

The results using this method shows then that where there is a 10percent pressure increase the sensor material volume changes willdecrease with additional loading so that if the additional loadincreases by say 50 kilopascals, then an additional 50 kilopascals ofchange in sensor pressure, which is the equivalent of soil matricpotential, will be required.

In FIG. 17, there again is shown a relationship discovered for a furthermaterial namely polyacrylamide showing the volume changes for appliedtotal pressures.

As will now be seen, the invention can be variously applied and variousmaterials are useful.

Reference has been made to three hydrogels which can be usefullyincorporated in to the concept of this invention.

Other examples of hydrogels which are considered to be useful in theapplications include an ultra-high molecular weight copolymer of2-hydroxyethyl methacrylate and N-vinyl pyrolidone and in another case,a copolymer of 2,3-dihydroxypropyl methacrylate and 2-hydroxyethylmethacrylate.

Preferred hydrogels have characteristics that allow them to be used overa significant period of time in the application and provide a sufficientresponse within the most useful range of from 50 kilopascals to 300kilopascals.

Some hydrogels might not be suitable in an actual application where theyare going to be exposed to bacterial attack where they might need tolast for some longer period of time in a damp environment.

The output from any of the described embodiments can have electricalcontacts for effecting electrical switching.

I claim:
 1. A matric potential responder including comprising a housing,which is sealed, apart from one part of the housing being porous toprovide for the effect of matric potential within soil to transfertherethrough, a liquid absorbing swellable non-liquid material selectedto exhibit an increase or decrease in expansive pressure in response toany matric potential of the hydraulically connected materials, saidswellable non-liquid material held with the housing under apredetermined range of compressive pressures and output means adapted toeffect an electrical output in response to changes in magnitude ofpressure exerted by said material when compressed.
 2. A matric potentialresponder as in claim 1 wherein the liquid absorbing swellablenon-liquid material is a hydrogel.
 3. A matric potential responder as inclaim 2 wherein the hydrogel is taken from the group consisting of apolyacrylate and an ultra-high molecular weight homopolymer of2-hydroxyethyl methacrylate.
 4. A matric potential responder as in claim3 wherein the hydrogel is selected from the group consisting of (a) anultra-high molecular weight copolymer of 2-hydroxyethyl methacrylate andN-vinyl pyrolidone and (b) a copolymer of 2,3-dihydroxypropylmethacrylate and 2-hydroxyethyl methacrylate.
 5. A matric potentialresponder as in claim 2 wherein the hydrogel is in one piece.
 6. Amatric potential responder as in claim 2 wherein the hydrogel is aplurality of pieces that can be filled into an internal chamber tocollectively take on a shape complementary to a shape of the internalchamber.
 7. A matric potential responder as in claim 1 wherein theoutput means includes a transducer in the form of a piezo resistor.
 8. Amatric potential responder as in claim 7 wherein the piezo resistor isprovided with a support that bows under pressure exerted by thehydrogel, the piezo resistor giving an output corresponding to thedegree of bowing.
 9. A matric potential responder as in claim 1 whereinthe porous part of the housing is a baked and unglazed ceramic material.10. A matric potential responder as in claim 1 wherein the materialselected and the compressive pressure applied are such that theresponder will provide an output to detected matric potential at leastwithin a range of from 50 kilopascals to 300 kilopascals.
 11. A matricpotential responder as in claim 1 wherein the output means includes apair of alternately openable and closable electrical contacts.
 12. Amatric potential responder as in claim 1 wherein the housing is rigidand the swellable non-liquid material is constrained, at least after theinitial expansion, to a substantially fixed volume over an entire rangeof matric potentials over which the responder operates.
 13. A matricpotential responder as in claim 1 wherein the housing is rigid and theswellable non-liquid material is constrained, at least after the initialexpansion, to a substantially fixed volume over an entire range ofmatric potentials over which the responder operates.
 14. A matricpotential responder comprising: a substantially sealed housing having aporous part for enabling a transfer into the housing of a matricpotential existing within ambient soil; a liquid absorbing swellablenon-liquid material selected to exhibit an increase or decrease inexpansive pressure in response to a change in the matric of the ambientsoil, said swellable non-liquid material being held with the housingunder a predetermined range of compressive pressures; and an electricalcircuit disposed in said housing in effective contact with saidswellable nonliquid material for generating an electrical output inresponse to changes in magnitude of pressure exerted by said swellablenon-liquid material.
 15. A matric potential responder as in claim 14wherein the liquid absorbing swellable non-liquid material is ahydrogel.
 16. A matric potential responder as in claim 15 wherein thehydrogel is taken from the group consisting of a polyacrylate and anultra-high molecule molecular weight homopolymer of 2-hydroxyethylmethacrylate.
 17. A matric potential responder as in claim 15 whereinthe hydrogel is selected from the group consisting of (a) an ultra-highmolecular weight copolymer of 2-hydroxyethyl methacrylate and N-vinylpyrolidone and (b) a copolymer of 2,3-dihydroxypropyl methacrylate and2-hydroxyethyl methacrylate.
 18. A matric potential responder as inclaim 15 wherein the hydrogel is in one piece.
 19. A matric potentialresponder as in claim 15 wherein the hydrogel is a plurality of piecesintroduceable into an internal chamber to collectively take on a shapeof the internal chamber.
 20. A matric potential responder as in claim 14wherein the electrical circuit includes a transducer in the form of apiezo resistor.
 21. A matric potential responder as in claim 20 whereinthe piezo resistor is provided with a support that bows under pressureexerted by the hydrogel, the piezo resistor giving an outputcorresponding to the degree of bowing.
 22. A matric potential responderas in claim 14 wherein the porous part of the housing is a baked andunglazed ceramic material.
 23. A matric potential responder as in claim14 wherein the swellable non-liquid material and the compressivepressure applied are such that the responder will provide an output todetected matric potential at least within a range of from 50 kilopascalsto 300 kilopascals.
 24. A matric potential responder as in claim 14wherein the electrical circuit includes a pair of alternately openableand closable electrical contacts.